Aqueous n-butyl acrylate copolymer dispersions for use as laminating adhesives

ABSTRACT

The present invention provides aqueous polymer dispersions comprising at least one particulate polymer P having a glass transition temperature T g  of less than 0° C. which is composed of ethylenically unsaturated monomers M including: 
     i. from 60 to 94.9% by weight, preferably from 75 to 89.5% by weight, of n-butyl acrylate as monomer M1, 
     ii. from 5 to 39.9% by weight, preferably from 10 to 24.5% by weight, of at least one monomer M2 selected from esters of methacrylic acid with C 1  to C 4  alkanols, tert-butyl acrylate, and vinylaromatic monomers, and 
     iii. from 0.1 to 5% by weight, preferably from 0.5 to 2% by weight, of at least one monomer M3 selected from ethylenically unsaturated compounds having at least one acid group; 
     the proportions of said monomers M1 to M3 being based on 100% by weight of monomers M, 
     obtainable by free-radical aqueous emulsion polymerization of monomers M in the presence of at least 0.01% by weight, based on the weight of the monomers M, of at least one molecular weight regulator. The present invention also provides for the use of such polymer dispersions in adhesive formulations for producing laminates, a process for producing laminates, and the laminates obtainable by this process.

The present invention relates to aqueous polymer dispersions and totheir use in aqueous adhesive formulations for producing laminates.

The use of aqueous polymer dispersions as adhesives and in aqueousadhesive formulations is known to the skilled worker. They have theadvantage over solvent-based adhesives that it is possible in principleto avoid solvent waste and solvent emissions.

In practice, adhesives for producing laminates, which are also referredto as laminating adhesives, have to meet a large number of differentrequirements. For example, laminating adhesives are desired to beuniversally applicable; in other words, they should be equally suitablefor the bonding of different polymer films made, for example, frompolyethylene (PE), oriented polypropylene (OPP), polyamide (PA) orpolyethylene terephthalate (PETP) with one another, with aluminum foilsor metallized polymer films, and for bonding polymer films with paper.

The laminating adhesives should have good substrate adhesion and afterlamination should bring about a high and durable level of strength inthe resulting laminates (film composites). Furthermore, a high level ofinstant strength of the film composite is desirable in order to permitrapid further processing, especially in the case of multi-ply laminates.Processing in the laminating units, furthermore, requires thedispersions to be of high shear stability and have good flow properties.

The aqueous polymer dispersions available commercially to date still donot go far enough toward meeting the requirements to which laminatingadhesives are subject.

Wo 92/12213 and EP-A 622 434 disclose laminating adhesives based onaqueous polymer dispersions comprising in copolymerized form both atleast one ethylenically unsaturated carboxylic acid and at least oneethylenically unsaturated sulfonic acid. Because of this acidcombination, such adhesives lead to comparatively high laminatestrength. Ethylenically unsaturated sulfonic acids, however, arecomparatively expensive and in many countries do not have approval underfood law, so that replacement of these monomers in polymers forlaminating adhesives is desirable.

DE-A 196 49 383 and the application DE 197 38 185.5, whose priority dateis earlier than that of the present specification, describe aqueouspolymer dispersions based on alkyl acrylates and their use as laminatingadhesives. The adhesives described result in good instant strength ofthe laminates.

The application DE 19816742, whose priority date is earlier than that ofthe present specification, discloses aqueous laminating adhesiveformulations whose polymers carry anhydride groups. For optimum bonding,amino-containing crosslinkers must be added to the dispersions duringformulation. Two-component adhesive formulations of this kind (2Ksystems) are of course more awkward to prepare than those consisting ofthe polymer dispersion as the only adhesive component (1K systems).Furthermore, 2K systems are often not sufficiently stable on storage.

It is an object of the present invention to provide aqueous polymerdispersions which, as laminating adhesives, bring about improvedlaminate strength relative to the prior art without the need forcrosslinkers or monomers containing sulfonic acid groups.

We have found that this object is achieved and that aqueous polymerdispersions composed essentially of n-butyl acrylate lead toparticularly stable laminates having good permanent and instant strengthif the polymers are prepared in the presence of small amounts of apolymerization regulator.

The present invention accordingly provides aqueous polymer dispersionscomprising at least one particulate polymer P having a glass transitiontemperature T_(g) of less than 0° C. Whiz is composed of ethylenicallyunsaturated monomers M including:

i. from 60 to 94.9% by weight, preferably from 75 to 89.5% by weight, ofn-butyl acrylate as monomer M1,

ii. from 5 to 39.9% by weight, preferably from 10 to 24.5% by weight, ofat least one monomer M2 selected from esters of methacrylic acid with C₁to C₄ alkanols, tert-butyl acrylate, and vinylaromatic monomers, and

iii.from 0.1 to 5% by weight, preferably from 0.5 to 2% by weight, of atleast one monomer M3 selected from ethylenically unsaturated compoundshaving at least one acid group,

the propportions of said monomers M1to M3 being based on 100% by weightof monomers M,

obtainable by free-radical aqueous emulsion polymerization of monomers Min the presence of at least 0.01% by weight, based on the weight of themonomers M, of at least one molecular weight regulator. The presentinvention also provides for the use of such polymer dispersions inadhesive formulations for producing laminates, and the correspondinglaminating adhesives.

Among the polymer P dispersions of the invention, preference is given tothose obtainable by polymerizing the monomers M in the presence of atleast 0.05% by weight and not more than 0.5% by weight, preferably inthe presence of from 0.1 to 0.5% by weight, in particular from 0.15 to0.4 and especially from 0.2 to 0.3% by weight, based on 100 parts byweight of the monomers M to be polymerized, of at least one molecularweight regulator. It is supposed that even small amounts of molecularweight regulators suppress the polymer crosslinking reactions thatalways take place to a, minor extent in the course of free-radical,addition polymerization. In general, the K value of the polymers P inthe latex obtained does not exceed a value of 90 (K value according toFikentscher determined in a 1% by weight solution of the polymer intetrahydrofurane.

Typical molecular weight regulators are organic sulfur compounds,halogenated hydrocarbons, silanes, allyl alcohols, and aldehydes.Molecular weight regulators given preference in accordance with theinvention are compounds having at least one thiol group such asthioglycolic acid, ethyl thioglycolate, mercaptoethanol,mercaptopropyltrimethoxysilane, and linear or branched alkyl mercaptanssuch as tert-butyl mercaptan and tert-dodecyl mercaptan. The regulatoris added to the polymerization vessel preferably continuously during thepolymerization of the monomers M. Preferably, both the major amount ofthe monomers M to be polymerized and the major amount of the molecularweight regulator are supplied continuously to the polymerizationreaction. The molecular weight regulator is preferably suppliedcontinuously to the polymerization reaction in the form of a separate,preferably aqueous solution or together with the monomers, e.g., in anaqueous monomer emulsion.

The vinylaromatic monomers specified as monomers M2 include styrene,a-methylstyrene, ortho-chlorostyrene, and vinyltoluene. The esters ofmethacrylic acid with C₁-C₄ alkanols include methyl methacrylate, ethylmethacrylate, isopropyl methacrylate, n-butyl methacrylate, isobutylmethacrylate, and tert-butyl methacrylate.

Preferred monomers M2 are methyl methacrylate and styrene.

Monomers M3 include monomers which contain at least one acidic group andthe anhydrides and salts of such monomers. Monomers M3 includeα,β-monoethylenically unsaturated mono- and dicarboxylic acids,monbesters of α,β-monoethylenically unsaturated dicarboxylic acids, theanhydrides of said α,β-monoethylenically unsaturated carboxylic acids,and also ethylenically unsaturated sulfonic acids, phosphonic acids ordihydrogen phosphates and the water-soluble salts thereof, e.g., theiralkali metal salts. Preferred monomers M3 are α,β-monoethylenicallyunsaturated C₃-C₈ carboxylic, acids and C₄-C₈ dicarboxylic acids,examples being itaconic acid, crotonic acid, vinylacetic acid,acrylamidoglycolic acid, acrylic acid and methacrylic acid, and also theanhydrides thereof. Particularly preferred monomers M3 are acrylic acidand methacrylic acid.

In addition to the abovementioned monomers M1to M3 the monomers M mayalso include further, auxiliary monomers. Examples of these includeneutral or nonionic, modifying monomers M4 of increased solubility inwater, e.g., the amides or the N-alkylolamides of the abovementionedcarboxylic acids, examples being acrylamide, methacrylamide,N-methylolacrylamide and N-methylolmethacrylamide, and also thehydroxyalkyl esters of the abovementioned α,β-monoethylenicallyunsaturated carboxylic acids, such as hydroxyethyl acrylate,hydroxyethyl methacrylate, hydroxypropyl acrylate and hydroxypropylmethacrylate. Further suitable auxiliary monomers M4 are the nitriles ofα,β-monoethylenically unsaturated C₃-C₈ carboxylic acids, such asacrylonitrile or methacrylonitrile. The monomers M generally includeless than 5% by weight, in particular less than 2% by weight, ofmonomers M4. The total amount of monomer M3 and monomer M4 is typicallyless than 5% by weight based on the total monomer amount.

Furthermore, the polymers P may also include bifunctional monomers M5,containing not only an ethylenically unsaturated double bond but also atleast one glycidyl or carbonyl group. Examples of monomers M5 areethylenically unsaturated glycidyl ethers and glycidyl esters: forexample, vinyl, allyl and methallyl glycidyl ether, glycidyl acrylateand glycidyl methacrylate, the diacetonylamides of the abovementionedethylenically unsaturated carboxylic acids, e.g.,diacetone(meth)acrylamide, and the esters of acetylacetic acid with theabovementioned hydroxyalkyl esters of ethylenically unsaturatedcarboxylic acids, an example being acetylacetoxyethyl (meth)acrylate.Monomers M5, like the anhydrides of α,β-monoethylenically unsaturatedmono- and dicarboxylic acids specified as monomers M3, allow thesubsequent crosslinking of the polymers P of the invention with, forexample, polyfunctional amines, hydrazides or alcohols.

Furthermore, a portion of the n-butyl acrylate can be replaced by otheresters of acrylic acid with C₁-C₁₀ alkanols or with C₅-C₁₀cycloalkanols, such as ethyl acrylate, 2-ethylhexyl acrylate andcyclohexyl acrylate, the fraction of these acrylates other than n-butylacrylate and tert-butyl acrylate being less than 20% by weight and, inparticular, less than 10% by weight based on the total monomer amount.

The polymer P is preferably composed exclusively of the monomers M1, M2and M3. The monomers M3 preferably include no anhydrides ofα,β-monoethylenically unsaturated mono- and dicarboxylic acids. In oneparticularly preferred embodiment the polymer P is composed of:

i. from 70 to 94.9% by weight, in particular from 80 to 89.5% by weight,of n-butyl acrylate,

ii. from 5 to 29.9% by weight, in particular from 10 to 19.5% by weight,of methyl methacrylate and/or styrene, and

iii. from 0.1 to 5% by weight, in particular from 0.5 to 2% by weight,of α,β-monoethylenically unsaturated monocarboxylic acid, especiallyacrylic acid and/or methacrylic acid,

the weight fractions of the monomers adding up to 100% by weight.

The glass transition temperature T_(g) of the polymers P present in thedispersions of the invention is preferably in the range from −60° C. to−10° C., in particular in the range from −50° C. to −15° C., andespecially in the range from −40° C. to −20° C. In this context itproves useful to estimate the glass transition temperature T_(g) of thedispersed polymer. According to Fox (T.G. Fox, Bull. Am. Phys. Soc.(Ser. II) 1, 123 [1956] and Ullmanns Enzyklopadie der technischenChemie, Weinheim (1980), pp. 17, 18), for the glass transitiontemperature of copolymers at high molecular masses, it holds in goodapproximation that$\frac{1}{T_{G}} = {\frac{X^{1}}{T_{g}^{1}} + \frac{X^{2}}{T_{g}^{2}} + {\ldots \quad \frac{X^{n}}{T_{g}^{n}}}}$

where X¹, X², . . . , X^(n) are the mass fractions of the monomers 1, 2,. . . , n and T_(g) ¹, T_(g) ², . . . , T_(g) ^(n) are the glasstransition temperatures of the homopolymers of each of the monomers 1,2, . . . , n in degrees Kelvin. The latter are known, for example, fromUllmann's Encyclopedia of Industrial Chemistry, VCH, Weinheim, Vol. A 21(1992) p. 169 or from J. Brandrup, E. H. Immergut, Polymer Handbook3^(rd) ed, J. Wiley, New York 1989.

It has also been found advantageous for the polymer particles of thepolymer P in the polymer dispersions of the invention to have an averageparticle diameter in the range from 50 to 1000 nm (as determined usingan ultracentrifuge or by photon correlation spectroscopy; on particlesize determination see W. Mächtle, Angew. Makromolekulare Chemie 185(1984), 1025-1039 and W. Mächtle, op. cit. 162 (1988), 35-42). In thecase of formulations with high solids contents, e.g., >50% by weightbased on the total weight of the formulation, it is advantageous ongrounds of viscosity for the weight-average particle diameter of thepolymer particles in the dispersion to be at least 100 nm. The averageparticle diameter will preferably not exceed 600 nm. It has also beenfound advantageous for the particle diameters of the individual polymerparticles to vary over a wide range, and in particular for the sizedistribution to have two or more maxima (polymer dispersions havingbimodal or polymodal polymer particle size distribution). Measures toadjust the polymer particle size distribution are known to the skilledworker (see, for example, EP-A 614 922 and documents cited therein).

The solids content of the polymer dispersions of the invention istypically in the range from 30 to 75% by weight and preferably in therange from 40 to 70% by weight. For the use in accordance with theinvention it is of advantage if the solids content is as high aspossible, i.e., at least 50% by weight.

The aqueous polymer dispersions employed in accordance with theinvention are prepared in accordance with the invention by free-radicalaqueous emulsion polymerization of said monomers M in the presence ofthe aforementioned amounts of at least one molecular weight regulator.The emulsion polymerization is customarily conducted in an aqueouspolymerization medium in the presence of at least one free-radicalpolymerization initiator and, if desired, of a surface-active substance.

Suitable free-radical polymerization initiators in principle includeboth peroxides, such as hydrogen peroxide, organic peroxides such astert-butyl hydroperoxide, alkali metal and ammoniumperoxodisulfates, forexample, and azo compounds. It is preferred to use redox initiatorsystems, which are composed of at least one organic reducing agent andat least one peroxide and/or hydroperoxide, an example being tert-butylhydroperoxide and a sulfur compound, such as the sodium salt ofhydroxymethanesulfinic acid, sodium sulfite, sodium disulfite, sodiumthiosulfate or acetone bisulfite adduct, or hydrogen peroxide withascorbic acid. For this purpose it is also possible to use redoxinitiator systems containing a small amount of a metal compound which issoluble in the polymerization medium and whose metallic component isable to exist in a plurality of valence states, an example beingascorbic acid/iron(II) sulfate/hydrogen peroxide, in which the ascorbicacid can in many cases also be replaced by the sodium salt ofhydroxymethanesulfinic acid, acetone bisulfite adduct, sodium sulfite,sodium hydrogen sulfite or sodium disulfite, and the hydrogen peroxideby organic peroxides, such as tert-butyl hydroperoxide, alkali metalperoxodisulfates and/or ammonium peroxodisulfate. Likewise preferredinitiators are peroxodisulfates, such as sodium peroxodisulfate orammonium peroxodisulfate. Preferably, the amount of free-radicalinitiator systems employed, based on the total amount of monomers to bepolymerized, is from 0.1 to 2% by weight.

The initiator can either be all included in the initial charge to thepolymerization vessel or else added continuously or in stages at therate at which it is consumed in the course of the free-radical aqueousemulsion polymerization. In each individual case it will depend, in amanner known to the skilled worker, both on the chemical nature of theinitiator system and on the polymerization temperature. Preferably, aportion is included in the initial charge and the remainder is suppliedto the polymerization vessel at the rate at which it is consumed.

Surface-active substances suitable for conducting the emulsionpolymerization are the emulsifiers and protective colloids commonlyemployed for such purposes. The surface-active substances arecustomarily used in amounts of up to 10% by weight, preferably from 0.1to 5% by weight and, in particular, from 0.5 to 4% by weight, based onthe monomers to be polymerized. The surface-active substances do ofcourse remain in the polymer dispersions and are therefore also aconstituent of the laminating adhesives of the invention.

Examples of suitable protective colloids are polyvinyl alcohols, starchderivatives and cellulose derivatives, or vinylpyrrolidone copolymers. Adetailed description of further suitable protective colloids can befound in Houben-weyl, Methoden der organischen Chemie, Volume XIV/1,Makromolekulare Stoffe [Macromolecular substances], Georg-Thieme-Verlag,Stuttgart 1961, pp. 411-420. Mixtures of emulsifiers and/or protectivecolloids can also be used. As surface-active substances it is preferredto employ exclusively emulsifiers, whose relative molecular weights,unlike those of the protective colloids, are usually below 2000. Theycan be anionic, cationic or else nonionic in nature.

The anionic emulsifiers include alkali metal and ammonium salts of alkylsulfates (alkyl: C₈-C₁₂), of dialkyl esters of sulfdsuccinic acid(alkyl: C₄-C₁₀), of sulfuric monoesters with ethoxylated alkanols (EOunits: 2 to 50, alkyl: C₁₂ to C₁₈) and with ethoxylated alkylphenols (EOunits: 3 to 50, alkyl: C₄-C₁₀), of alkylsulfonic acids (alkyl: C₁₂-C₁₈)and of alkylarylsulfonic acids (alkyl: C₉ to C₁₈). The anionicsurface-active substances also include mono- and dialkyl derivatives ofsulfonylphenoxybenzenesulfonic salts, especially their sodium, potassiumor calcium salts. The alkyl groups in these compounds generally have 6to 18 and especially 6, 12 or 16 carbon atoms. Use is frequently made oftechnical mixtures comprising a proportion of from 50 to 90% by weightof the monoalkylated product. These compounds are common knowledge, forexample, from U.S. Pat. No.4,269,749, and are obtainable commercially,for example, as Dowfax® 2A1 (trademark of the Dow Chemical Company).

Suitable nonionic emulsifiers are araliphatic or aliphatic nonionicemulsifiers, examples being ethoxylated mono-, di- andtrialkylphenols.(EO units: 3 to 50, alkyl: C₄-C₉), ethoxylates oflong-chain alcohols (EO units: 3 to 50, alkyl: C₈-C₃₆), and alsopolyethylene oxide/polypropylene oxide block copolymers. Preference isgiven to ethoxylates of long-chain alkanols (alkyl: C₁₀-C₂₂, averagedegree of ethoxylation: 3 to 50) and, of these, particular preference tothose based on oxo alcohols and naturally occurring alcohols having alinear or branched C₁₂-C₁₈ alkyl radical and a degree of ethoxylation offrom 8 to 50.

Further suitable emulsifiers can be found in Houben-Weyl, Methoden derorganischen Chemie, Volume XIV/1, Makromolekulare Stoffe [macromolecularsubstances], Georg-Thieme-Verlag, Stuttgart, 1961, pp. 192-208.

The surface-active substances used to prepare the polymer dispersions ofthe invention preferably include at least one anionic emulsifier. It hasbeen found advantageous for the stability of the polymer dispersions ofthe invention, especially to mechanical loads such as shear forces, forthe anionic emulsifiers preferably used to prepare the dispersions ofthe invention to include at least one salt of a dialkyl ester ofsulfosuccinic acid (linear or branched C₄-C₁₀ and, in particular, C₈alkyl radical), preferably an alkali metal salt and in particular thesodium salt.

The emulsion polymerization takes place in general at from 30 to 130,preferably from 50 to 90° C. A K value <90 can also be achieved bypolymerization at temperatures above 80° C., preferably above 90° C.and, in particular, above 100° C.

The polymerization medium can consist either of water alone or else ofmixtures of water with water-miscible organic liquids such as methanol,ethanol, n-propanol, isopropanol, n-butanol, tert-butanol,tetrahydrofuran, formamide, or dimethylformamide, the proportion of saidliquids usually being less than 10% by weight based on thepolymerization medium. It should be borne in mind here that the presenceof alcohols during the polymerization may result in a reduction inmolecular weight. Preferably, water alone is used as polymerizationmedium.

The emulsion polymerization can be conducted either as a batch processor in the form of a feed process, including stages or a gradientprocedure. Preference is given to the feed process, in which themonomers in pure or emulsified form are supplied to the polymerizationzone continuously, in stages or under a concentration gradient, with thepolymerization being maintained. The individual components can be addedto the reactor, in the case of the feed process, from above, from theside or from below, through the reactor floor.

Besides the seed-free preparation route, a defined polymer particle sizecan be established by conducting the emulsion polymerization by the seedlatex process or in the presence of seed latex prepared in situ.Corresponding processes are known and can be found in the prior art (seeEP-B 40 419, EP-A-614 922, EP-A-567 812 and literature cited therein andalso ‘Encyclopedia of Polymer Science and Technology’, Vol. 5, JohnWiley & Sons Inc., New York 1966, p. 847).

In the case of the seed latex process, the polymerization is customarilyconducted in the presence of from 0.001 to 3% by weight and, inparticular, from 0.01 to 1.5% by weight of a seed latex (solids contentof the seed latex, based on total monomer amount), preferably with seedlatex included in the initial charge (initial charge seed). The latexgenerally has a weight-average particle size of from 10 to 100 nm and,in particular, from 20 to 50 nm. Examples of its constituent monomersare styrene, methyl methacrylate, n-butyl acrylate and mixtures thereof,it being possible for the seed latex to include in copolymerized form aminor fraction of monomers M3 and/or M4 as well, preferably less than10% by weight based on the total weight of the polymer particles in theseed latex.

In order to remove the residual monomers it is common following thepolymerization to carry out physical deodorization, by distilling offthe volatile monomers with steam, for example, or chemicaldeodorization, in which case, after the end of the emulsionpolymerization proper—that is, after a monomer conversion of at least95%, or after the residual monomer content has been lowered to a level<5% by weight by physical deodorization—further initiator is added, suchas a redox initiator, for example.

The polymer dispersions of the invention are used in accordance with theinvention in aqueous adhesive formulations for producing laminates;i.e., in aqueous laminating adhesive formulations for bonding substratesof large surface area. The present invention therefore also provides aprocess for producing laminates in which an aqueous adhesive formulationis used which comprises at least one of the polymer dispersions of theinvention. In this context, the aqueous polymer dispersions can be usedas they are or after formulation with customary auxiliaries. Examples ofcustomary auxiliaries are wetting agents, thickeners, protectivecolloids, light stabilizers, and biocides. With the adhesiveformulations of the invention there is no need to add plasticizingresins (tackifiers) or other plasticizers.

In the process of the invention for producing laminates, the polymerdispersion of the invention, or an appropriately formulated preparation(formulation), is applied to the substrates of large surface area thatare to be bonded, preferably in a layer thickness of from 0.1 to 20,with particular preference from 1 to 7 g/m² by means, for example, ofknife coating, brushing, etc. After a short time for evaporation of thedispersion water (preferably after from 1 to 60 seconds), the coatedsubstrate can then be laminated with a second substrate, in which casethe temperature can be, for example, from 20 to 200, preferably from 20to 100° C. and the pressure can be, for example, from 100 to 3000,preferably from 300 to 2000 kN/m².

Examples of suitable substrates are polymer films, especially those ofpolyethylene (PE), oriented polypropylene (OPP), polyamide (PA),polyethylene terephthalate (PETP), polyacetate, cellophane, polymerfilms (vapor-)coated with metal (e.g., aluminum) (metallized films forshort) or else paper, card or metal foils, such as those of aluminum.Said foils or films can be bonded to one another, to another substrate,or to a foil or film of a :different type, e.g., polymer films to metalfoils, polymer films to paper, different polymer films to one another,etc. Said foils and films can also be printed with printing inks, forexample.

The polymer dispersions of the invention can be used both in adhesiveformulations for high-gloss film lamination and in adhesive formulationsfor composite film lamination. In the case of high-gloss filmlamination, paper or card is bonded to transparent polymer films. In thecase of composite film lamination, the abovementioned substrates (butnot paper or card), such as different polymer films, can be bonded toone another.

An advantage of the invention is that substrates of very different kindscan be bonded to one another, i.e., laminated, with the polymerdispersions of the invention ensuring good adhesion of the adhesiveformulation to the substrates and resulting in high strength of thebonded composite. Furthermore, the polymer dispersions of the inventionare notable for high shear stability.

EXAMPLES I. Preparation of The Polymers P as Aqueous Dispersions D1 toD5 and CD1 to CD4

A polymerization reactor was charged with 116 g of deionized water and0.28 g of polystyrene seed polymer (in the form of an aqueousdispersion; d₅₀=30 nm) and this initial charge was heated to 85° C. 10%of the initiator feed was added to the initial charge, during which thetemperature was maintained. After 5 minutes, during which thetemperature was maintained, the monomer feed and initiator feed wereboth added to the polymerization reactor over the course of 180 minutes,beginning simultaneously. The polymerization temperature was thereaftermaintained for a further 30 minutes. Then at 85° C. 5.6 g of a 10%strength by weight aqueous tert-butyl hydroperoxide solution and 7.5 gof an aqueous solution of the sodium bisulfite adduct of acetone (12%strength) were added. The mixture was subsequently cooled to roomtemperature and the dispersion was neutralized to a pH of 4 to 5 using15% strength by weight sodium hydroxide solution. The solids content ofthe dispersion was approximately 54 to 56% by weight.

Monomer feed: aqueous emulsion of

275.0 g of deionized water

560.0 g of monomers (see Table 1)

2.24 g of emulsifier solution 1

7.47 g of emulsifier solution 2

y g of tert-dodecyl mercaptan (t-DMC; see Table 1)

Emulsifier solution 1: 60% strength by weight aqueous solution ofbis-2-ethylhexylsulfosuccinic acid sodium salt

Emulsifier solution 2: 45% strength by weight aqueous solution ofdodecylphenoxybenzenedisulfonic acid sodium salt (DOWFAX® 2A1 fromDOWCHEMICAL)

Initiator feed: solution of

2.8 g of sodium peroxodisulfate in

37.2 g of water

TABLE 1 Disper- n-BuA EHA MMA MA AA t-DMC D²⁾ Tg³⁾ sion [%]¹⁾ [%] [%][%] [%] [%] [nm] [° C.] CD1 84 — 15 — 1 0 293 −27 D1 84 — 15 — 1 0.10274 −28 D2 84 — 20 — 1 0.15 292 −29 D3 84 — 15 — 1 0.20 278 −30 D4 84 —15 — 1 0.25 296 −31 D5 84 — 15 — 1 0.30 288 −32 CD2 42 42 15 — 1 0.30305 −39 CD3 84 — — 15 1 0.30 279 −35 n-BuA = n-butyl acrylate, EHA =2-ethylhexyl acrylate, MMA = methyl methacrylate, MA = methyl acrylate,t-DMC = tert-dodecyl mercaptan, AA = acrylic acid ¹⁾% by weight based on100% by weight of monomers ²⁾Average particle diameter determined usinga Malvern Autosizer 2c, Malvern Instruments, England, on 0.01% strengthby weight samples. ³⁾Glass transition temperature (DSC, midpoint)

II. Performance Testing of Dispersions D1-D5 and CD1 to CD3

Preparation of The Composite Films:

The neutralized polymer dispersions were knife-coated in a dry layerthickness of 2-3 g/m² onto various commercially customary films(polyethylene=PE, d=100 pm, manufacturer 4P-Folien, Forchheim (DE);polypropylene, Corona-pretreated on one side=PP, d=33 am; polyethyleneterephthalate=PETP, d=12 μm; aluminum, d=15 μm, manufacturers F.A.

Universal Alufolien, D. H. Korff). After drying with hot air, the filmscoated in this way were rolled up together with a second film (see Table2) and subsequently pressed in a roller press under a pressure of 6.5bar at 5 m/min and at 70° C. The composite films were subsequentlystored at room temperature for 1 day under standardized climaticconditions.

b) Determination of peel strength

For this purpose the composite films obtained in accordance with a) werecut into strips 15 mm wide. The strips were subsequently subjected topeeling at 23° C. in a universal peel strength tester machine from Zwick(model 1120.25.01) at a rate of 100 mm/min and an angle of 180°, and theforce required for this (in newtons) was measured. The results arecompiled in Table 2.

TABLE 2 Peel strength [N/15 mm] PP/ Example Dispersion PE/PP PETP (met)PETP/Al C1 CD1 1.2 0.7 1.2 1 D1 1.6 1.0 1.7 2 D2 1.4 1.2 1.5 3 D3 1.40.9 1.7 4 D4 2.2 1.6 2.5 5 D5 1.5 1.1 2.2 C2 CD2 1.1 0.7 1.1 C3 CD3 0.60.4 0.7 PE = polyethylene PP = polypropylene Al = aluminum PETP =polyethylene terephthalate PETP(met) = Al-metallized polyethyleneterephthalate

We claim:
 1. A process comprising polymerizing a monomer mixture in thepresence of at least 0.01% by weight of at least one molecular weightregulator to form an aqueous polymer dispersion of a particulate polymerhaving a glass transition temperature Tg of less than 0° C., wherein thepolymerization is an aqueous emulsion polymerization carried out in areaction vessel and the molecular weight regulator is added continuouslyto the reaction vessel during the polymerization, wherein the monomermixture consists essentially of M1 from 60 to 94.9% by weight of n-butylacrylate, M2 from 5 to 39.9% by weight of at least one of a C₁ to C₄alkanol ester of methacrylic acid, tert-butyl acrylate, or avinylaromatic monomer, and M3 from 0.1 to 5% by weight of at least oneethylenically unsaturated compound having at least one acid group,wherein % by weight is based on 100% by weight of the monomer mixture.2. The process as claimed in claim 1, wherein the molecular weightregulator is an organic compound having at least one thiol function. 3.The process as claimed in claim 1, wherein the monomers M3 are selectedfrom the group consisting of α,β-ethylenically unsaturatedmonocarboxylic acids, α,β-ethylenically unsaturated dicarboxylic acidsand mixtures thereof.
 4. The process as claimed in claim 1, wherein themonomer M2 is methyl methacrylate.
 5. The process as claimed in claim 1,wherein the glass transition temperature Tg is from −60° C. to −10° C.6. The process as claimed in claim 1, wherein the glass transitiontemperature Tg is from −50° C. to −15° C.
 7. The process as claimed inclaim 1, wherein the glass transition temperature Tg is from −40° C. to−20° C.
 8. The process as claimed in claim 1, wherein the monomer M1 isn-butyl acrylate present in an amount of from 70 to 94.9% by weight, themonomer M2 is methyl methacrylaie, styrene, or both methyl methacrylateand styrene, present in an amount of from 5 to 29.9% by weight, and themonomer M3 is acrylic acid, methacrylic acid, or both acrylic acid andme thacrylic acid, present in an amount of from 0.1 to 5% by weight. 9.The process as claimed in claim 1, wherein the polymerization is carriedout in the presence of a surface-active substance present in an amountof from 0.1 to 5% by weight.
 10. The process as claimed in claim 1,wherein polymerizing includes continuously feeding the monomer mixturein pure or emulsified form to the reaction vessel.
 11. The process asclaimed in claim 1, wherein the polymerization is carried out in thepresence of a seed latex.
 12. The process as claimed in claim 11,wherein the seed latex is present in an amount of from 0.01 to 1.5% byweight.
 13. The process as claimed in claim 1, wherein the monomers M1,M2 and M3 do not contain sulfonic acid groups.
 14. The process asclaimed in claim 1, wherein the molecular weight regulator and themonomer mixture are supplied continuously and separately to the reactionvessel.
 15. The process as claimed in claim 1, wherein thepolymerization is carried out in the presence of a free-radicalpolymerization initiator.
 16. The process as claimed in claim 15,wherein free-radical polymerization initiator is a peroxide.
 17. Theprocess as claimed in claim 16, wherein the peroxide is an organicperoxide.
 18. The process as claimed in claim 15, wherein a portion ofthe free-radical polymerization initiator is included with an initialcharge of the monomer mixture and the molecular weight regulator in thereaction vessel, and the remainder of the free-radical polymerizationinitiator is supplied to the reaction vessel at the rate at which it isconsumed.
 19. The process as claimed in claim 1, further comprisingneutralizing the polymer dispersion to a pH of from 4 to
 5. 20. Theprocess as claimed in claim 1, wherein the molecular weight regulator istert-dodecyl mercaptan.
 21. The process as claimed in claim 1, whereinthe monomer mixture consists of monomers M1, M2 and M3.